The above-and-below format (also known as the over-and-under format), as taught in U.S. Pat. No. 4,523,226 to Lipton et al., has become the basis for a number of stereoscopic software applications and hardware embodiments. The simplicity of the format and its universality with regard to computers and video accelerator boards makes it an outstanding choice for a stereoscopic format. With reference to FIG. 2 (to be discussed more fully below), we divide the image into two portions (subfields): one above (203) and the other below (207), separated by an added horizontal blanking area (205). With this arrangement, we can create an image disposition in which spatial juxtaposition at the normal field rate can be transformed into temporal juxtaposition at twice the field rate by the introduction of a synchronization pulse midway between the top and bottom of the field within the added blanking area.
In the above-and-below format, we create two subfields. (Whether the left image is on the top or the right is on the top is arbitrary, and of no significance in this discussion.) The above-and-below format uses these subfields to incorporate left and right images, in other words, a stereo pair. The subfield blanking interval is created between the subfields, and it is into this subfield blanking interval that a synchronization pulse is added. Therefore, the software developer or content provider need only produce images that are above and below each other, separated by this artificial, or added, subfield vertical blanking area or interval.
The images are squeezed in the vertical anamorphically by a factor of approximately two. We say “approximately two” because a certain number of lines are taken up in housekeeping, namely, in the subfield blanking area, so the number is not exactly two, but it is a close enough approximation. Once we have a properly formatted subfield image as described here, we can add a subfield vertical blanking pulse by some means located between the computer and the monitor.
This means can count video lines or in some way tell the distance between the top and the bottom of the field, in order to properly time the injection of a synchronization pulse into the vertical blanking subfield. Once the added pulse reaches the monitor, the monitor looks at the pulse and triggers the monitor's refresh based on the added vertical blanking pulses. This added pulse alternates with the original vertical blanking pulse. Thus, as mentioned above and will be discussed below, the monitor runs at twice the usual refresh rate. Let us say the video board is running at 60 fields per second. With the added pulse, the monitor will run at 120 fields per second. It must pay attention to the added vertical blanking pulses as well as the original vertical blanking pulses. The reader can now understand the reason for anamorphically compressing the image by a factor of two. This compression will become decompressed when running full-screen images on the monitor, and the normal proportions of the images will be restored.
The major virtue of the above-and-below format is that it is video-accelerator-board independent. It requires no additional software driver support of any kind, as required by other systems, to make the video board work at a high field rate in the stereoscopic mode. The high field rate is a requirement for flicker-free imaging. Each eye must see the same number of images it had seen in the planar mode, or about sixty fields per second.
The above-and-below format also intrinsically indexes or tags the left and right fields with regard to perspective content, which is important since the viewer must see a stereoscopic rather than a pseudostereoscopic image. A pseudostereoscopic image is one in which the left eye sees the right image and vice versa.
The format intrinsically accomplishes distinguishing between left and right image perspectives by employing a standard, for example, by always placing the left image in the top subfield. In this way, we are able to tell that the subfield which immediately follows an original vertical blanking pulse (and not the added pulse) is tagged as the left image.
Above-and-below format images are observed with individual selection devices, as is typically the case for plano-stereoscopic displays. These selection devices may be of the active type, such as CrystalEyes® or StereoEyes™ wireless eyewear that use an infrared communications link to establish synchronization of the eyewear's liquid crystal shutters with the writing of the subfields. Such a link consists of an infrared emitter, usually located at or near the monitor, and a receiver located within the eyewear. The shutters in the eyewear open and close out of phase with each other but in synchrony with the original or added sync pulses. Additionally, wired eyewear may be used, the eyewear receiving power by some means other than internal batteries.
Another means for viewing the above and below plano-stereoscopic images is a passive selection device using polarizing eyewear. In this case, a liquid crystal modulator is mounted in close proximity to the monitor, and it changes the characteristics of polarization in synchrony with the view field rate.
In effect, the above-and-below format is a topological transformation in which, as has been mentioned, a spatial juxtaposition is turned into a temporal juxtaposition by means of adding a synchronization pulse between the subfields. The system works because a cathode-ray-tube monitor pays attention to synchronization pulses in order to write a new field and will intrinsically restore the image to its normal aspect ratio while doubling the field rate. However, it may be necessary to calibrate, by some means, the distance between the subfields' center-to-center distance for a particular graphics card because of inconsistencies in the way graphics cards manage the inactive lines within the vertical blanking interval. There is no standard duration for a vertical blanking interval for computers, as there is, by contrast, for television. This is a disadvantage of the present above-and-below system.
A particular graphics card may support a number of different display resolutions (for example, 640×480, 1024×768, and 1280×1024). For a particular resolution, there may be a number of valid vertical refresh rates (for example, 60 Hz, 72 Hz, and 85 Hz). Even with the same resolution and refresh rate, the graphics card may have a number of “modes” whereby the graphics card will mimic the exact timing of a number of other devices or industry standards. While the active or visible display area is a constant at, say 640×480, the horizontal, and even more importantly for the purposes of this discussion, the vertical blanking and sync periods will vary. The application software has available to it only the information about the active area; information about the blanking interval is unavailable.
For the above-and-below format to work correctly, the added vertical sync pulse must be applied exactly between the sync pulses generated normally by the graphics card. The application software must then place the left subfield image in the same position relative to the actual vertical sync pulse as the right subfield image is placed relative to the inserted vertical pulse. Without additional information, it is impossible for the application software to determine this location.
Another problem that is associated with the above-and-below format, at least for a mass communications medium, is that in its historical embodiment, the user must flip a switch to add the vertical synchronization pulse between the subfields. In other words, the system will not automatically turn on in stereo. The electronics that add the subfield sync pulse have to be manually instructed to do so, which is not bothersome for the professionals who presently use the format. However, as long as the user must add the subfield blanking vertical synchronization pulse, the usefulness of the system is restricted to professional users, users who will take the care and trouble to actually flip a switch. Therefore to facilitate ease of use, it will be important to use a signifier to automatically sense the presence or lack of a stereo signal and adjust the user's display accordingly.
A field flag detector, similar to this signifier, has been described in U.S. Pat. No. 5,572,250, in which a blue line code or index is added to the bottom of each field. This blue-line coding system is well-suited to identifying the “handedness” of a field, i.e., whether it is a left eye image or a right eye image. Graphics cards used in such a system alternate images, that is to say, the image on the screen is already flipping between left and right views. The purpose of the detector is to relay the status to the selection device (the eyewear). The downside of an erroneous decoding is minimal. If a non-stereo image is misinterpreted as a stereo pair, the user merely takes off the eyewear.
The ramifications of a misinterpreted signifier used in the above-and-below format are much more serious. If a non-stereo image is detected as a stereo pair, and the external sync doubling device adds the additional vertical sync pulse, the image on the screen is unusable. The top and bottom halves of the image are overlaid and look like a photographic double-exposure. For example, it could not be discerned whether a cursor or pointer visible on the screen was in the upper or lower half. In addition, any image located within the vertical subfield blanking interval will, on most monitors, generate a distracting diagonal line during the retrace. For all of these reasons, it is especially important that the signifier be robust and immune to false detection.
In this disclosure, we describe a signifier used in combination with the above-and-below format to provide a unique and a foolproof system which is ideal for video games and similar mass-consumer products.
In addition, the signifier overcomes one of the problems of the format, as described above, namely that a graphics card by graphics card calibration may be necessary to properly center the two subfields. If the subfields are displayed by the monitor in an uncentered mode, then the result will be vertically offset images which will produce vertical screen parallax, which in turn will cause the user's eyes to converge in the vertical direction. Convergence in the vertical direction does not occur when looking at the visual field and is an artifact of improperly setup stereoscopic display systems. The result for the user is eyestrain or fatigue and a general loss of enjoyment of the displayed image.
Our signifier, by properly compensating for each and every graphics card, notwithstanding the particular operating parameters of any display mode or resolution, overcomes this vexing prior art problem. There are some systems presently in use which use a full-frame indexing scheme, and these cannot overcome the vertical misalignment problem described above. However, the signifier (or index) described below, entirely overcomes this problem.
The signals available to the signifier are the red, green and blue analog video and the digital horizontal and vertical synchronization signals. A signifier code can be devised that will be located with the horizontal or vertical blanking periods, but for the important fact that these areas are not available to the application or driver software. It is a rare instance where the hardware allows information to be placed within either blanking period. Thus, the signifier code must appear within the visible area of the screen, since that is the only practical option available. The other alternative, equally formidable, is to change the video “housekeeping” standards to allow for the inclusion of a left/right index or signifier. There is no likelihood of obtaining changes to an industry standard, given that such a change might adversely affect monitor performance.